What Makes a Roof Coating Cool?
There are two properties of cool roof coatings which keep roof surfaces cool in the sun: • High reflectivity to reflect away the sun’s energy instead of absorbing it. • High emissivity to radiate away any energy they do absorb. Cool coatings have solar reflectivities of 60-75%, while conventional roofing materials have reflectivities of only 5-25%. Most roofing materials have emissivities which are a healthy 90% or higher. Metals are one unfortunate exception, with low emissivities that range between 20-60% depending on their surface finish and condition. Even though they’re generally very reflective, metal roofing and metallic roof coatings (i.e. aluminum coatings) tend to get hot since they’re unable to emit away even the modest amount of solar energy they do absorb (think “Cat on a Hot Tin Roof”). The effect of these two properties on roof surface temperature is striking. Conventional roof surfaces with low reflectivity and high emissivity heat to 160-190 F at midday during the summ
